Biomass boiler efficiency

Biomass boiler efficiency in general has come under some criticism in a recent Guardian article .

Key points from the Guardian article are that a DECC study of installed biomass systems found that the average efficiency of the boilers was 66.5%, that the surveyed systems can only reach 76% efficiency, and that this is lower than the needed 85% efficiency to contribute to carbon targets. Yet the original research also states  “In other words, the estimated central performance standards of 81.5% to 72% (and 76.75% on average) would only be slightly exceeded if the boiler were gas, oil or coal fired.”

Whilst the article was clearly pushing heat pumps, a rival renewable technology, there seems to be confusion in the text.

To qualify for the Renewable Heat Incentive biomass boilers have to conform to a European Standard EN303-5. In addition to this the boilers have to be tested to emit limited emissions.

The 85% efficiency relates to the EN303-5 standard and the class of boiler that can be used for an RHI Scheme. This is calculated in lab conditions from the burning of a specific fuel. This is not the overall efficiency of the heating system.

Over the last 2 years there are a number of variables that may have influenced the study.

Issue 1 – Heat Meters

Heat meters are used to measure effective heat that is used by the RHI biomass system. What is recorded on the heat meter is what is paid to the owner of the system. In the majority of cases the heat meters are placed on the entrance to the heated spaces. Thus they only measure what is being used, and not what is being wasted. In addition there have been some issues with the installation and accuracy of the meters, where the accuracy can vary by up to 30%. There are currently free courses to help remedy this.

Issue 2 – Fuel

When the RHI first came into being the boilers could be tested on a very efficient fuel, but in reality the boilers were using a similar fuel that was much less efficient.

For instance you can buy wood briquettes that can have a low moisture content e.g. 8%. These will burn very efficiently, in excess of 90% in the right conditions. However if you use logs that are not properly seasoned then the moisture content will be much higher e.g. 30%, and the efficiency of the burning process may be as low as 65%.

This issue has been addressed in part by a tightening of the specifications. However there are boilers installed that are burning poor quality fuel as it is cheaper than specified fuel. This is more prevalent with landowners or farmers. Today all newly installed boilers should only be burning specified fuel and the boilers should be able to burn with an efficiency of above 85%. Owners of the installation have to keep records of the fuel they have purchased.

Wood Pellets all have to be ENplus. Regular samples of pellets are taken and they are measured for moisture, dust (fines), hardness, and burn quality. There have been some issues with dust in particular. The pellets can degrade in transit and when loaded into a hopper. There are examples where the dust content can rise above 10 or 15%. Clearly this makes the efficiency of fuel to heat produced less efficient. Again these issues are often dealt with by an experienced installer, or many of the good distributors will offer replacement pellets if they have been wrongly handled. There are very few issues with bagged ENplus pellets.

The majority of the boilers in the study seemed to be using woodchip at 199kW. In the case studies provided the fuel used was often wood chips with a moisture value of 30 or 31 percent. Wood Chips are significantly cheaper than wood pellets, but burn less efficiencty, and therefore will have a poorer efficiency. For instance if the burn efficiency of 31% moist pellets is 80% then the heat transfer to water will be below 80%. This will largely explain the number of responses that seemed to be 70% + and fairly well installed.

Issue 3 – sizing of boilers

With Solar PV everyone who purchased wanted as much as possible. This produces more electricity, which is a good thing! For biomass the owner of a system gets paid the highest tariff up to 199kW. So if you are buying a biomass system for investment purposes then it seems that choosing 199kW will earn you more money.

The difficulty with this is that this causes 2 problems. a) The boiler is not run at its most efficient output. This is true, for instance, for many combination condensing boilers that have been sized for hot water production and not heating need. The boiler never reaches its optimum output and is less efficient as a consequence. b) The heat is not really needed. Here the owner is more interested in the tariff rather than the efficient distribution of the heat. The heat is then lost.

Issue 4 – UK weather and system management

UK weather presents a number of issues. Firstly we do have mild winters and cold winters. This can relate to the sizing issue. When you size a boiler you examine possible temperatures that you want the boiler to be able to cope with. For many this may be -3 degrees. In reality we may experience the majority of the winter with temperatures of between 6-12 degrees.

The other issue with UK weather is that it is intermittent. We generally do not need the heating on all day every day. Therefore you have to consider carefully when the heating needs to be turned on. If the following month is then warm you will wast the heat that is already in your system. For instance if you have buffer tanks and have spent time heating them up, only not to use the heat, then this heat could be lost and not used.

Issue 6 – General heating system efficiency

What is not clear from the Guardian article is how the efficiency is measured. The original article is far clearer and looks at the whole system efficiency rather than implying that the efficiency is all down to the boiler.

Before the heat gets to the heat meter there may be pipework (lagged), district heating pipe work, buffer tanks, and the transfer of the heat from the burn chamber within the boiler to hot water.

Examining the transfer from heat to water, there is a UK database that is slowly expanding that shows that heat to water efficiency (typically 80-86%) from biomass boilers rather than the burnt efficiency of the burning process that is often above 90%.

Due to the nature of the study being more about 199kW boilers for predominantly large retail systems with possibly wood chip boilers, it does not cover smaller systems that do not have district heating, or many pellet boilers.

If you are in doubt over what your system efficiency is or even means, then please get in touch with us.

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